A precise airway tone is critical for maintaining proper air pressure during inspiration and preventing airway collapse during expiration. Neonates with disorders such as tracheomalacia, a condition where the trachea is floppy, develop breathing difficulties soon after birth. In more common respiratory diseases such as cystic fibrosis, recent studies have revealed that structural defects in the upper airway precede and may contribute to the increased susceptibility to infections. Normal airway tone is achieved through a precise balance of cartilage and smooth muscle that provide the rigidity and elasticity, respectively. In addition, evidence suggests that cartilage and smooth muscle play a key role in maintaining stem cells in the subjacent airway epithelium, and patterning the vasculature and nerves in the surrounding airway mesenchyme. The fundamental questions of how the balance between cartilage and smooth muscle is established, and what their specific roles are as organizers of other tissues in the microenvironment remain poorly understood. We have generated a series of cartilage and smooth muscle mutants, and will use them to test the in vivo requirements of these lineages in homeostasis, responses to airway injury and allergen challenges.